they shouldn’t be moling with an 11 KV across the line, they such expose the cable and hand dig that section.
Moling can be “erratic” and assumes that rock isn’t present
I don’t believe you can hold them to ‘ransom’,
in regard to a water service pipe, yes you can
ask yourself whether you ever envisage doing anything in the area of the proposed pipe
I would as a minimum get your own solicitor to review and get them to pay those costs, secondly the presence of the pipe adds value to their house (which they are selling so there is no ongoing need to be “nice”)and devalues your land and you need to ensure this is recognised